1. Field of the Invention
This invention relates to communication systems and particularly to a receiving system for substantially cancelling undesired broadband noise in the same frequency spectrum as a desired signal.
2. Prior Art Statement
Many different prior art devices, apparatuses and receiving systems have been proposed for separating, removing cancelling noise or interference from an input signal.
The article of Bernard Widrow et al, entitled "Adaptive Noise Cancelling: Principles and Applications", found on pages 1692-1716 of Proceedings of the IEEE, Vol. 63, No. 12, Dec. 1975, teaches the concept of adaptive noise cancelling. The adaptive noise cancelling concept, as shown in FIG. 1 on page 1693, uses a two input device to which is respectively applied a primary input containing a signal corrupted by additive noise or interference, and a reference input containing noise correlated in some unknown way with the primary noise. The reference input is adaptively filtered and subtracted from the primary input to obtain a resultant estimate of the signal. Basically, Widrow et al is a time-waveform canceller.
U.S. Pat. No. 4,052,559 discloses a one input, speech filtering device which utilizes a transversal filter to provide an estimate of only the longer correlation time-period "noise" that is contained in an input speech-bearing signal and a subtractor to subtract the longer correlation time-perod "noise" from the input speech-bearing signal to obtain substantially only the shorter correlation time-period speech component of the input speech-bearing signal as the signal remnant. Unlike the device of Widrow et al, the speech filtering device of U.S. Pat. No. 4,052,559 is a linear predictor which predicts the correlated noise component that is contained in the input speech-bearing signal.
U.S. Pat. No. 3,126,449 basically discloses a nonlinear audio noise blanker, which detects noise bursts and interrupts the passage of audio during the noise bursts. More specifically, this patent discloses a noise discriminator circuit having three parallel branches, each branch consisting of a different band pass filter in series with an integrator-detector/Schmitt trigger circuit combination. The outputs of the three parallel branches are selectively applied to various logic gates to develop a control signal which, by way of a one-shot multivibrator and a holdover circuit, controls an audio gate for the purpose of accomplishing the noise discrimination function.
U.S. Pat. No. 3,803,357 discloses a noise filter device which operates as an automatic noise canceller based on audio band segmentation and selective gain recombination. The noise filter device comprises a plurality of gain controllable, contiguous, narrow band nonlinear filters connected to a signal source, a noise tracker and a summing amplifier. The noise tracker is also connected to the signal source and essentially controls the ability of each of the narrow band filters to pass a signal as a function of the noise level that the noise tracker detects in the signal. The summing amplifier combines the spectral outputs of the narrow band filters in the proper power phase relationship.
U.S. Pat. No. 3,953,802 discloses an adjacent channel rejector for use in communications systems. Basically, this adjacent channel rejector is a nonlinear noise blanker. In this patent the IF signal of the receiver is applied through an IF amplifier and demodulator to an inhibit gate. The IF signal is also applied to a high gain amplifier. The output of the high gain amplifier is applied through a narrow band filter/demodulator to a first input of an amplitude comparator, as well as through a wide band filter/demodulator to a second input of the amplitude comparator. When a received signal has a center frequency outside a predetermined channel bandwidth, the high gain amplifier is driven into saturation, subsequently causing the comparator to apply an inhibit signal to the inhibit gate to prevent the received signal from passing through the inhibit gate.
U.S. Pat. No. 3,769,591 discloses a frequency selective pulse receiving system which performs a function similar to that performed by the system of U.S. Pat. No. 3,953,802. This pulse receiving system comprises a main channel having a pass band to receive incoming signals, three auxiliary channels respectively having pass bands equal to, above, and below the desired pass band of the receiver, and a comparator circuit responsive to the output signals of the three auxiliary channels for controlling the operation of a suppression device to either pass or suppress the output of the main channel as a function of the amplitude relationships of the output signals of the three auxiliary channels. In this manner the system automatically notches out interference.
Each of the above-described systems disclosed in U.S. Pat. Nos. 3,126,449; 3,802,357; 3,953,802; and 3,769,591 achieves noise cancellation by distorting, blanking, inhibiting or otherwise modifying the desired signal, as well as the noise, whenever noise is detected. As a result, none of these systems could properly operate in the presence of intense and continuous broadband noise.
None of the above-described prior art published article and U.S. patents teaches or suggests an adaptive noise cancelling receiver which includes the combination of: a circuit for selecting a desired broadband frequency signal containing a desired signal; first and second frequency channels responsive to the broadband frequency signal for respectively generating first and second audio signals, with the first audio signal containing the desired signal in the presence of a first broadband noise and the second audio signal containing a second broadband noise that is correlated with the first broadband noise; an adaptive filter responsive to the second broadband noise and to an output signal for adaptively developing an estimate of the first broadband noise; and a combiner for combining the estimate of the first broadband noise with the first audio signal to substantially cancel out the first and second broadband noises and develop as the output signal a signal which substantially contains only the desired signal.